Solder bonding is a critical step in many industrial processes, e.g., in interconnection and packaging of electronic devices. The most widely used solder is the near eutectic Pb-60% (by weight) Sn alloy. However, due to the toxicity of lead (Pb), there is currently substantial interest in Pb-free substitute solder compositions. Some such compositions are known, (see, for instance, W. B. Hampshire, Electronic Materials Handbook Vol. 1, Packaging, ASM International, Metals Park, Ohio, 1989, p.633).
However, known Pb-free solder compositions have melting temperatures that differ significantly from that of the standard Pb-Sn solder, and have not found significant use in, e.g. electronic packaging. It is common practice in electronic packaging to solder the various levels of the package with different solders of different melting points, selected so that the soldering of each successive level does not inadvertently melt the previously soldered level. Thus, use of a substitute solder having a melting point that differs substantially from that of the solder that is being replaced might require re-design of the packaging operation (a very expensive proposition), or else could cause problems in manufacturing sequences, efficiency, and/or yields. In view of the strong reasons for avoiding Pb-based solder, it would be very desirable to have available a Pb-free solder composition having a melting temperature close (e.g., within .+-.10.degree. C.) to that of the standard 40/60 Pb-Sn solder, and having other characteristics (e.g., strength, wettability, creep resistance) that make it suitable as a substitute solder for the standard Pb-based solder. This application discloses such a composition.